The present invention relates to a scanning optical system for forming an electrostatic latent image on a scan target surface such as a surface of a photoconductive drum.
As is well known, scanning optical systems are installed in a variety of printing devices such as laser beam printers, fax machines and copy machines. The scanning optical system dynamically deflects a laser beam (which has been modulated according to image information) by use of a deflecting system such as a rotating polygon mirror and converges the dynamically deflected laser beam on the surface (the scan target surface) of the photoconductive drum by use of an imaging optical system, by which the scan target surface is scanned with a spot beam and thereby an electrostatic latent image composed of a plurality of dots are drawn on the scan target surface.
In general, intensity distribution of the laser beam incident on the scan target surface is not a perfect Gaussian distribution, and it is known that the main beam is accompanied by several rings of light (side lobes) of lower light quantity which are caused by diffraction at apertures placed on the optical path of the laser beam. The side lobe is known to expose the photoconductive drum and cause print error called “black stripes” in halftone printing if the intensity of the side lobe exceeds approximately 6% of the central intensity of the main beam (reference: Japanese Patent Provisional Publication No. HEI09-080333). In an ideal state of the imaging optical system, intensities of side lobes remain at approximately 4% of the central intensity of the main beam, by which the black stripes are not caused.
However, if microscopic undulations exist on an optical surface of the imaging optical system, the intensity of the side lobe changes if the laser beam passes through the undulating part. In this case, if the intensity of the side lobe exceeds the threshold value, the black stripes occur in halftone printing.